This invention relates to a process for purifying non-aqueous fluids. More particularly, this invention relates to the removal of ionizable impurities from non-aqueous liquids.
Separations of small quantities of acid and basic gases from neutral gases are frequently performed in industry. Examples of these separations include removal of hydrogen sulfide from natural gas, and other fuel or process gas streams; cleanup of waste gas streams containing oxides of sulfur and nitrogen; and recovery of ammonia from waste gases of industrial or agricultural sources. Similarly the recovery of catalysts and removal of ionic impurities from organic liquid products and solvents is widely practiced.
Methods presently used in the art for gas purification include the use of selective liquids and absorbent solids with affinity for acids or bases that are contacted with the feed gas. The efficient contacting of the liquid with the gas and the recovery of the absorbed acid or basic gas are significant process problems requiring the application of energy and equipment in multiple process steps. Other processes use high pressure and selective membranes to separate the gases by diffusion through the membrane. These processes have a removal efficiency dependent on the pressure difference across the membrane. Therefore, efficient separation and recovery of low levels of gaseous impurities is not economically attractive.
Similarly, methods for the removal of ionizable impurities from non-aqueous liquids suffer from the same shortcomings as those used in gas purification.
Present commercial practices for the removal of catalyst and other impurities from polyols include the following:
Nitzeki et al, in U.S. Pat. No. 3,528,920, published Sep. 15, 1970, describe the use of synthetic magnesium silicate as an adsorbent for the removal of an alkaline catalyst from glycol ethers. However, the method requires the neutralization of the alkaline catalyst acid prior to filtration.
The removal of inorganic catalysts from polyols using an electrostatic precipitator is described in U.S. Pat. No. 3,582,491, published Jun. 1, 1971, by J. F. Louvar et al. A mixture of polyol, water and a solvent which is immiscible in water and having a density substantially different from water is formed. The solvent is employed in an amount sufficient to adjust the density differential between the polyether-solvent solution and water to at least 0.03 gram per milliliter. The polyether-solvent solution is then separated from the water by electrostatic coalescence. A stream of water containing water-soluble impurities and a stream of containing polyether-solvent solution are separately recovered after the precipitation, followed by separating the solvent from the polyether-solvent solution. This method is not attractive due to high processing costs.
Ion exchange techniques have also been employed for removal of the catalyst from polyols. In this technique, the crude polyol, mixed with water and or an organic solvent, is passed through a column packed with ion exchange resin to remove the ionic impurities. However, few types of resins can produce polyols having ionic impurities at less than about 5 parts per million and having acceptable color.
The use of ion exchange resins are described in Polish patent Nos. 54624 and 62214 published Mar. 8, 1969 and Mar. 20, 1971 respectively, by T. Sniezik et al to remove catalysts from polyether polyols. For basic catalysts such as potassium hydroxide, a highly acidic cation exchange resin is used. The purification process rate is dependent on the temperature and the molecular weight of the polyether. The polyol is removed from the ion exchange resin by washing with water, preferably containing a non-ionic surfactant. The ion exchange resin is then regenerated with acid or alkali.
Japanese patent publication No. 51-23211, published Feb. 24, 1976 by Y. Uchiyama refines alkoxylates of alcohols or polyalkylene glycols containing alkali catalysts by passing the alkoxylate through a type H regenerated cation exchange resin.
These processes are time consuming as they require low flow rates of the order of 1 bed volume per hour. Further, the processes have waste disposal problems associated with the regeneration process. When used with an organic solvent, ion exchange is costly as solvent stripping is required along with frequent resin regeneration.